Trying to shoot northern lights, able to see images in my view finder eye piece, dark on live view and the pics turned out totally black. I used ISO 800-1000, 8 sec, F3.5 manual mode.
Typical settings for northern lights are f2.8 or higher, 30 seconds and 1600-3200 iso. Also it helps if you have a full frame camera, thus you'll have less noise.
30 seconds and fast aperture and high ISO are the reason to get the shot fast enough. If your camera gets less light then you'll have to exposure longer and it will cause stars to leave a trail, because they always move.
Also shoot RAW so you can enhance your images later.
When you get a black image, no matter what you are photographing you basically have the same 3 (or sometimes 4) options (after checking you didn't leave the lens cap on, obviously)
Increase ISO - but be aware that this will also increase noise (but you can deal with that at post)
Use a slower shutter speed - of course this is likely to cause motion blur and make moving/changing thing invisible (or at least somewhat transparent)
Open up the aperture - here you pay with a thinner DOF, you didn't say what lens you are using but I guess that at f/3.5 you are already wide open with a kit lens
not relevant for northern lights, but I'm adding this here for completeness - add light
So, since you are already maxed out on aperture, you can't add light and northern lights are moving there's actually only one option left - ISO.
I would boost the ISO until I see something in the image (even if it's mean getting it too high to get a useable image), when I get something I would then play with ISO and shutter speed (lowering ISO and slowing down the shutter by the same number of stops) until I get something I like.
Able to see images in my view finder eye piece
When looking through viewfinder, light which is entering your objective is being reflected from mirror, then from the prism so you can see it. Following picture is showing you this process:
So that's why you can see your scene in your viewfinder.
When you switch to live view, part 2 and 6 move, so the light hits your image sensor 7, and you can see your image on your LCD screen. Be careful, when you switch to live view, you can't see your scene through view finder, couse your mirror 2 is up.
Dark on live view and the pics turned out totally black
But why your pictures are dark? Your image sensor is not exposed properly, that means your camera settings are not good, and not enough light hits your sensor (7).
Your aperture is wide open, and that's OK. Try to change your shutter speed, or ISO.
If you change your shutter speed to 16 seconds (your shutter speed was 8 seconds) you will double the amount of light that will hit the sensor. If your picture is still dark, then double your ISO settings. For exampe set ISO to 2000 or 4000, and hit shutter release button. Note that with this settings, amount of light that will hit your sensor is 4 times higher, and your will se changes.
Also, you should get familiar with light meeter in your viewfinder. Light meter is processing part in your camera, which tells you how current picture settings will affect your image sensor. If your settings are bad, you will get under exposed picture (your case), or over exposed picture (bright picture). You can see value of light meter in your viewfinder. Following picture is showing your light meter (item 19).
If there are lines on the left of zero value, that means that your picture is little bit over exposed. If there are lines on the right of the zero value, that means that your picture is under exposed little bit, so you should expose your image sensor with more light. This is default settings on Nikon. So try it right now, see how it works.
I suggest you to watch videos about shutter speed, ISO and aperture, and you will get much better pictures if you understand how things are working.